This invention relates to a method of sensing an excessive slip condition for a vehicle slip control system.
When a torque is applied to a vehicle wheel for braking or driving the wheel, a force is generated between the wheel and the road surface tending to decelerate or accelerate the vehicle. This force is dependent upon various parameters, including the road surface conditions and the amount of slip between the wheel and the road surface. As the torque input to the wheel is increased, the force between the tire and the road surface increases as wheel slip increases, until a critical slip value is surpassed. When the slip exceeds this critical slip value, the force between the wheel and the road surface decreases. Further, as wheel slip increases, the lateral forces acting on the slipping wheel decrease.
Various methods have been proposed for preventing this excessive slip condition of the wheels of a vehicle. In the case of acceleration slip, these methods include a traction controller that adjusts engine torque and/or the application of the brakes of the driven wheels when an excessive slip condition is sensed. In the case of vehicle braking, a wheel-lock controller limits brake pressure when an excessive slip condition is sensed. Whatever method is selected for controlling wheel slip, it is an objective of these systems to (a) maximize the longitudinal driving or braking forces and (b) to continuously maintain lateral
In order to achieve the foregoing objectives, it is desirable to control entry into traction or anti-lock braking control so as to minimize premature entry to thereby achieve a maximum transfer of force between the wheels and the road surface while at the same time maintaining lateral stability.